International Journal of Sports Physiology and Performance, 2014, 9, 876-881 http://dx.doi.org/10.1123/ijspp.2013-0494 © 2014 Human Kinetics, Inc.
www.IJSPP-Journal.com ORIGINAL INVESTIGATION
Assessment of Bilateral Asymmetry in Cycling Using a Commercial Instrumented Crank System and Instrumented Pedals Rodrigo R. Bini and Patria A. Hume The accuracy of commercial instrumented crank systems for symmetry assessment in cycling has not been fully explored. Therefore, the authors’ aims were to compare peak crank torque between a commercial instrumented crank system and instrumented pedals and to assess the effect of power output on bilateral asymmetries during cycling. Ten competitive cyclists performed an incremental cycling test to exhaustion. Forces and pedal angles were recorded using right and left instrumented pedals synchronized with crank-torque measurements using an instrumented crank system. Differences in right (dominant) and left (nondominant) peak torque and asymmetry index were assessed using effect sizes. In the 100- to 250-W power-output range, the instrumented pedal system recorded larger peak torque (dominant 55–122%, nondominant 23–99%) than the instrumented crank system. There was an increase in differences between dominant and nondominant crank torque as power output increased using the instrumented crank system (7% to 33%) and the instrumented pedals (9% to 66%). Lower-limb asymmetries in peak torque increased at higher power-output levels in favor of the dominant leg. Limitations in design of the instrumented crank system may preclude the use of this system to assess peak crank-torque symmetry. Keywords: bicycle, torque, dominance, pedal forces Bilateral cycling motion has usually been assessed assuming symmetry in force production and kinematics of lower limbs. However, differences in power output and mechanical work of the legs have ranged from 5% to 20% in uninjured cyclists and noncyclists.1 Conflicting results were reported comparing cyclists2–4 and noncyclists5 without clear relationships between pedaling cadence2 and power-output level5 in bilateral symmetry. Peak torque at the propulsive phase of crank revolution (ie, from 12 o’clock to 6 o’clock crank positions) has been reported as one of the most important predictors of performance during 40-km time trials,6 given that a large percentage of the force applied to the pedal in the sagittal plane can be translated into crank torque in this part of crank revolution.7 Therefore, cyclists should apply large crank torque on both cranks to enhance power output for a given pedaling cadence. Using peak torque as a measure of pedaling symmetry, authors have reported that differences between legs were significant at lower power-output levels (≤90% of maximal oxygen uptake) and decreased at higher power-output levels (ie, >91% of peak oxygen uptake) for 6 competitive cyclists.3,8 In contrast, another study did not show significant differences in mean torque computed during full crank revolution for 11 cyclists at different power-output levels (60–100% of maximal oxygen uptake).4 Therefore, it is unclear if crank-torque symmetry is related to power-output level. The potential reduction in asymmetries in torque at higher power-output levels may be due to an increased bilateral neural input by interhemispheric cortical communication to facilitate the excitability of both legs.1
Bini is with the Exercise Research Laboratory, Federal University of Rio Grande do Sul, Porto Alegre, Brazil. Hume is with Sport Performance Research Inst New Zealand, Auckland University of Technology, Auckland, New Zealand. Address author correspondence to Rodrigo Bini at bini. [email protected]. 876
Evaluation of bilateral asymmetry has gained popularity because some commercial devices provide right-to-left crank comparisons for torque and power output. One example is the SRM torque-analysis system, which enables the user to assess peak crank torque from dominant and nondominant lower limbs during cycling.9 The SRM power meter measures the deformation on the shafts of the crank set resulting from the torque applied on both cranks (ie, net crank torque). However, a recent study showed that the measures of peak torque from the SRM torqueanalysis system are only accurate for power output greater than 80% of maximal power output.10 Therefore, separate measures of dominant and nondominant crank torque are not accurate using this device, because the torques at dominant and nondominant cranks are computed as net torque (ie, torque from the contralateral leg diminishes torque from the ipsilateral leg). Using the commercial instrumented crank systems (ie, SRM torque-analysis system), it has been assumed that peak torque observed during the propulsive phase of crank revolution is exclusively affected by the ipsilateral leg,3 which may not be completely valid. Consequently, the accuracy of assessment of bilateral symmetry using instrumented crank systems may be compromised by the design of this device. Instrumented pedals have been able to provide crank-torque measurements independently for dominant and nondominant legs,11 which are expected to offer a more accurate measure of peak crank torque than instrumented crank systems. Therefore there is uncertainty on the accuracy of asymmetries in bilateral peak torque taken from instrumented crank systems (ie, SRM torque-analysis system). There is also a need to determine whether potential differences in bilateral peak-torque measurements taken from instrumented pedals and instrumented crank systems would be affected by changes in power-output level. Therefore, our aims were to compare peak crank torque between a commercial instrumented crank system and instrumented pedals and to assess the effect of power-output level in bilateral asymmetries during cycling. Our hypotheses were that the instrumented
Assessment of Bilateral Asymmetry in Cycling 877
crank system would underestimate bilateral asymmetries and that asymmetries would be diminished at higher power-output levels.
Methods Participants Ten cyclists (3 female and 7 male club riders) with competitive experience in cycling and/or triathlon were invited to participate in the study: age 30 ± 7 years, body mass 72.8 ± 13 kg, standing height 175 ± 12 cm, maximal oxygen uptake 55.6 ± 8.8 mL · kg–1 · min–1, peak power output 336 ± 77 W, and peak power per body mass 4.6 ± 6 W/kg). Lower-limb dominance assessed by the Waterloo Inventory indicated that all 10 cyclists were right-leg dominant. The ethics committee of AUT University approved the research protocol (AUTEC 10/56).
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Design A quantitative repeated-measures experimental design was used to collect data (cross-sectional).
Data Collection Force components (normal and anteroposterior) from a 2-dimensional pedal dynamometer custom developed for Look-type cleats12,13 were computed using the regression between 3 static load points (0, 5, and 10 kg) applied to the pedals and voltage output when R2 was greater than .99. Mechanical coupling between anteroposterior and normal loads was corrected using a gain matrix.14 Potentiometers attached to the pedal spindle (Vishay Spectrol model 357, Vishay Intertechnology, Malvern, PA, USA) were calibrated using a manual goniometer (Physio-Med Services, model 30 cm, Patterson Medical Ltd, Nottinghamshire, UK) set at 4 crank-to-pedal axle angles (0°, 90°, 180°, and 270°, taken twice for each crankto-pedal angle) to compute the relationship between voltage output and the measured angle. The calibration factors were defined when differences in voltage taken from both trials were lower or equal to 1% for each given crank-to-pedal angle. Body mass and height were measured according to International Society for the Advancement of Kinanthropometry protocols.15 Cyclists’ bicycle saddle height and horizontal position were measured to set up the stationary cycle ergometer (Velotron, Racermate Inc, Seattle, WA, USA). The cyclists performed an incremental cycling exercise on the cycle ergometer with 3 minutes of warmup at 100 W and pedaling cadence visually controlled at 90 ± 2 rpm. Power output was then increased to 150 W and continued increasing in a step profile of 25 W/min until cyclist exhaustion.16 A script was configured in the Velotron CS2008 software (Velotron, Racermate Inc, Seattle, WA, USA) for automatic control of the cycle-ergometer power output in a constant-workload mode. This configuration enabled a constant power output with cycleergometer resistance changing to balance for fluctuations in pedaling cadence. Gas exchanges were continuously sampled from a mixing chamber where samples were drawn into the oxygen and carbon dioxide analyzers for continuous measurement using a metabolic cart (TrueOne 2400, Parvo Medics, Salt Lake City, UT, USA). Analyzers for oxygen and carbon dioxide were calibrated according to manufacturer recommendations. Maximal power output and maximal oxygen uptake were defined as the highest power output measured during the test and as the highest oxygen uptake computed over a 15-second average of the data, respectively, for assessment of
cyclists’ performance and fitness level. All aforementioned procedures served as familiarization. After 2 to 7 days, cyclists returned to the laboratory at the approximate same time of day to perform the incremental test following the same procedures. Normal and anteroposterior forces were measured using a pair of strain-gauge instrumented pedals,12 with pedal-to-crank angle measured using angular potentiometers attached to the pedal spindle. Pedal-force signals passed through an amplifier (Signal conditioning unit, model RM-044, Applied Measurements, Mitcham, Australia) and, along with potentiometers, reed switch signals and SRM torque-analysis system signals (Schoberer Rad Meßtechnik, Jülich, Germany) were recorded using an analog-to-digital board (PCI-MIO-16XE-50, National Instruments, Austin, USA) at 600 Hz per channel using a custom-made script in Matlab (Mathworks Inc, Natick, MA, USA). Analog data were acquired between the 20th and the 40th seconds of each step of 50 W (ie, 100, 150, 200, 250 W, etc).
Data Analyses Pedal-to-crank angle measured by the potentiometers were converted into sine and cosine to compute tangential and radial forces on the cranks. A low-pass zero-lag Butterworth digital filter with cutoff frequency of 10 Hz was applied to the sine and cosine data from potentiometers to attenuate signal noise from the gap in potentiometer voltage readings.11 Crank torque was measured by the pedals using the tangential force on the cranks and crank length, as well as by the SRM torque-analysis system, as shown in Figure 1. A frequency-to-voltage conversion factor of 4 × 10–4 and frequency-to-torque factor gathered at the calibration trial were used to convert torque measurements from voltage to Nm. A reed switch attached to the bicycle frame detected the position of the crank in relation to the pedal revolution and enabled separate pedal forces and torque data for every crank revolution and for the propulsive (ie, from 12 o’clock to 6 o’clock crank positions) and recovery phases (ie, from 6 o’clock to 12 o’clock crank positions) for right and left cranks. Peak crank torques of right (dominant) and left (nondominant) cranks were determined when the crank was at the propulsive phase and at the recovery phase, respectively, using
Figure 1 — (A) Image of the instrumented pedal attached to the instrumented crank system. (B) Illustration of the locations of sensors for cranktorque measurement for the instrumented crank system and instrumented pedals. Vertical arrows indicate crank torque applied simultaneously by the ipsilateral and contralateral legs.
878 Bini and Hume
a clockwise motion of the crank as reference. Peak crank torque was averaged over 5 complete pedal revolutions for each crank on the instrumented pedals and the commercial instrumented crank system. Standard deviations for peak torque from dominant and nondominant limbs were computed from 5 crank cycles to report intralimb variability. Asymmetry index (AI%) was calculated as outlined by Robinson et al,17 using measures from dominant (D) and nondominant (ND) legs normalized by the average of dominant and nondominant measures:
AI% = {(D – ND)/[(D + ND/2]} × 100
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Statistical Analyses Errors of calibration of normal and anteroposterior components and potentiometers of the pedals were computed as average percentage differences in voltage due to calibration load (or angle for potentiometer) in relation to the output voltage. As an example, for the normal force of the right pedal, the difference in voltage from 0 kg to 5 kg was 0.1547 V, and the difference in voltage from 5 kg to 10 kg was 0.1544 V, resulting in 0.19% difference in voltage due to load application. Variation in pedaling cadence was computed by percentage differences across 5 crank revolutions. Peak torque for dominant and nondominant cranks and asymmetry index (mean and SD) were compared for instrumented pedals and the instrumented crank system. All variables were analyzed for the 100-W, 150-W, 200-W, 250-W, 300-W, and 350-W power outputs of the incremental test. Normality of distribution and sphericity were confirmed for all variables via the Shapiro-Wilk and Mauchly tests, respectively, after application of a logarithmic transformation using SPSS for Windows 16.0 (SPSS, New York, NY, USA). Mean percentage differences between dominant and nondominant peak torques and the asymmetry index from the instrumented crank system and the instrumented pedals were computed, and comparisons were conducted using Cohen effect sizes (ES). We used ESs opting for a threshold of large effects (ES = 1.0) for substantial
changes. This is a more conservative approach than previously described,18 but it would ensure a nonoverlapping in distribution of scores greater than 55%.19
Results Errors from calibration procedures were 0.19% and 0.68% for the normal force and 0.68% and 0.56% for anteroposterior force for the dominant and nondominant pedals, respectively. Error in pedal-to-crank angle of each potentiometer was 0.5%. Mean variation in pedaling cadence between cyclists was 1%, resulting in an estimated error from equipment of ~1.37% and ~1.74% for crank torque of the dominant and nondominant pedals, respectively. For the instrumented cranks, errors in instrumented crank measurements followed reports from previous studies.20 Greater peak torque was observed for dominant and nondominant pedals than for instrumented crank system, as shown in Figure 2 and Table 1. In general, large differences for dominant (31–48%) and nondominant (17–39%) peak crank torque between the commercial instrumented crank system and the instrumented pedals were observed at power-output ranges of 100 to 250 W. At higher power outputs (300 W and 350 W) there were moderate to trivial differences in peak crank torque comparing dominant with nondominant pedals. There was a trend for an increase in the difference between dominant and nondominant crank torques using the commercial instrumented crank system (7–33%) and the instrumented pedals (9–66%), but large differences were only found for the instrumented pedals at power outputs higher than 200 W. The instrumented pedals presented larger asymmetry indices than the commercial instrumented crank system at 250 W (see Table 1). Intralimb variability for peak crank torque from dominant and nondominant limbs (4–11%; see Table 2) was smaller than differences between dominant and nondominant peak torques for the instrumented crank system (5–33%) and for the instrumented pedals (9–66%).
Figure 2 — (A) dominant, (B) nondominant, and (C) dominant + nondominant crank torque measured by the pedals and by (D) the instrumented crank system (D). Data from 5 consecutive revolutions of the 10 cyclists at 200 W of workload and 90 rpm of pedaling cadence. Arrows indicate peak crank torque.
879
23 ± 8
31 ± 11
39 ± 14
53 ± 19
73 ± 25
100 W (n = 10)
150 W (n = 10)
200 W (n = 10)
250 W (n = 10)
300 W (n = 7)
350 W (n = 6)
D
75 ± 7
65 ± 8
56 ± 8
48 ± 7
41 ± 6
33 ± 5
CS vs D
3%; 0.1, T
19%; 0.9, M
31%; 1.6, L
36%; 2.0, L
43%; 2.4, L
48%; 3.0, L
CS
55 ± 18
45 ± 11
36 ± 9
28 ± 8
22 ± 7
18 ± 8
46 ± 14
46 ± 12
43 ± 12
39 ± 8
34 ± 10
30 ± 9
ND
21%; 0.6, M
1%; 0.1, T
17%; 0.7, M
27%; 1.3, L
35%; 1.4, L
39%; 1.4, L
CS vs ND
Nondominant Peak Torque (Nm)
33%; 0.8, M
17%; 0.5, M
8%; 0.3, S
10%; 0.3, S
5%; 0.1, T
7%; 0.2, T
CS
66%; 2.8, L
42%; 1.9, L
29%; 1.3, L
26%; 1.3, L
19%; 0.8, M
9%; 0.4, S
Pedals
Dominant vs Nondominant Peak Torque
Abbreviations: CS, crank system; D, dominant pedal; ND, nondominant pedal; T, trivial ES; S, small ES; M, moderate ES; L, large ES.
CS
17 ± 6
Power-output level
Dominant Peak Torque (Nm)
CS
27 ± 18
13 ± 20
5 ± 15
8 ± 17
4 ± 15
6 ± 17
51 ± 36
36 ± 33
28 ± 31
22 ± 30
20 ± 33
11 ± 28
Pedals
93%; 0.9, M
189%; 0.9, M
428%; 1.0, L
181%; 0.6, S
418%; 0.7, M
280%; 0.8, M
CS vs pedals
Asymmetry Index (%)
Table 1 Mean ± SD, Mean Percentage Differences, and Effect Sizes (ES) Comparing Both Systems (Commercial Instrumented Crank System and Instrumented Pedals) and Differences Between Dominant and Nondominant Cranks for Peak Torque and Asymmetry Index
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880 Bini and Hume
Table 2 Intralimb Variability in Crank Torque for the 6 Power Outputs From the Incremental Test From 10 Cyclists, Mean (SD) Peak torque, Nm Power output
Dominant
Nondominant
100 W (n = 10) 150 W (n = 10) 200 W (n = 10) 250 W (n = 10) 300 W (n = 7) 350 W (n = 6)
1.4 (4%) 2.2 (5%) 2.7 (6%) 3.4 (6%) 5.1 (8%) 8.1 (11%)
2.2 (7%) 1.8 (5%) 2.2 (6%) 3.2 (8%) 3.2 (7%) 3.7 (8%)
Note: SD for peak torque from 5 crank cycles for dominant and nondominant limbs.
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Discussion Our study compared dominant with nondominant peak crank torque measured by a commercial instrumented crank system (ie, SRM torque-analysis system) and instrumented pedals during bilateral cycling at varying power-output levels. The reason for these comparisons was based on the increasing use of the instrumented crank system for the assessment of asymmetries in cycling3,8 and potentially because the commercial instrumented crank system may underestimate peak crank torque and asymmetry.10 Lower peak torque was observed in this study for the commercial instrumented crank system compared with the instrumented pedals for the same power-output level. Greater asymmetries were observed at higher power-output levels, but large differences between dominant and nondominant crank torque were only observed using the instrumented pedals. The asymmetry index was greater than 20% (usually reported for uninjured cyclists1) for the commercial instrumented crank system only at 350 W and for the instrumented pedals at power outputs greater than 150 W. The primary reason for these differences between systems is related to the electronic characteristics of each system. The commercial instrumented crank system, as outlined in Figure 1, is designed to measure the deformation on the shafts of the crank set due to the torque applied on both cranks. Using the example highlighted in Figure 1, if a cyclist applied 20 Nm of torque (clockwise) with the ipsilateral leg and 5 Nm of torque (anticlockwise) with the contralateral leg, the commercial instrumented crank system would record 15 Nm of torque. Therefore, the torque at dominant and nondominant cranks is computed as a net torque (ie, torque from the contralateral leg diminishes torque from the ipsilateral leg). Instrumented pedals measure the force on the pedal surface (eg, normal and anteroposterior components) and compute crank torque independently using pedal-to-crank angles.11 This is a more accurate approach because forces from each leg are measured separately, reducing contralateral-to-ipsilateral effects on crank-torque measures. Therefore, care should be taken if instrumented cranks are used for pedaling-technique assessment due to possible interference in the data of each leg caused by the opposite leg, as previously conducted.21 Intralimb variability has been recently suggested to play a role in bilateral asymmetries in running.22 However, intralimb variability in peak torque (4–11%) was smaller in our study than differences between dominant and nondominant peak torque (5–66%). Additional concern has been raised due to the use of the asymmetry index to identify bilateral differences in kinetic variables during dynamic tasks.22 Large between-subjects variability in asymmetry indices (for the instrumented crank system and the instrumented
pedals) resulted in less pronounced ESs comparing instrumented cranks with pedals than using raw crank-torque data. Reduced ESs for comparison between instrumented cranks and pedal-asymmetry indices reinforce the limitation of asymmetry indices to detect bilateral differences in kinetic variables during cycling. Higher power outputs resulted in greater bilateral differences assessed by instrumented pedals that are contrary to previous findings.3,4,8 Cyclists were observed to reduce differences in peak torque at higher power outputs3,8; however, peak torque was measured using an instrumented crank system in those studies. Another study did not observe effects from power-output level in full-crankrevolution average torque symmetry in cyclists using instrumented pedals.4 Reductions in asymmetries in crank torque at higher power outputs have been hypothesized due to a potential increased bilateral neural input by interhemispheric cortical communication to facilitate the excitability of both legs.1 However, studies assessing muscle activation in cyclists during bilateral cycling exercise at increasing power outputs did not report differences in lower-limb muscle activation comparing both legs.23 Muscle activation during single-leg cycling did not differ when cyclists’ dominant and nondominant legs were compared,24 which suggests that lower-limb neural drive may not differ between legs. Substantial differences in cycling efficiency have not been observed in cyclists during single-leg cycling, suggesting that contributions to efficiency from independent legs are similar.24 Potential differences in joint motion25 from bilateral asymmetries in bone dimensions could be a reason for observed asymmetries in pedaling kinetics. It may be the case that asymmetries are related to changes in bone dimensions due to asymmetrical load applied to the skeleton during bone growth.26 Further research should shed light on why some cyclists present larger crank-torque asymmetries than others and how this would affect their performance and injury risk. Lower-limb dominance may have played a role in increasing asymmetries at higher power outputs. Evidence suggests that the kicking dominant leg contributed significantly more to average crank power than the nondominant leg,2 which is in line with our results. It has been previously shown that pedaling asymmetries are highly variable across different days of assessment and that asymmetries depend on exercise condition.2,5 Further research should assess bilateral muscle activation and joint kinetics in cyclists with similar levels of crank-torque asymmetry to compute individual joint contributions to crank torque from right and left legs. Some limitations may have affected the results of our study. Trials using single-leg cycling could have been conducted to isolate the influence of the contralateral leg in torque readings from the instrumented crank system. During single-leg cycling, torque measured by the instrumented crank system should not be affected by the contralateral leg; therefore, crank-torque measures would be isolated to the ipsilateral leg.27 Although this approach is not ecologically optimal, it would shed light on the effects of contralateral resistive torque on the data from the instrumented crank system and would provide a practical use of the instrumented crank system for measurement of bilateral asymmetries. Combined fatigue and power-output changes during the incremental test may have affected measures of peak torque and bilateral asymmetries. Randomizing power outputs would isolate for fatigue and work-rate effects in crank-torque asymmetries.
Practical Applications For coaches and cyclists, assessing bilateral asymmetries should preferably be conducted using instrumented pedals rather than instrumented crank systems at varying power-output levels. The
Assessment of Bilateral Asymmetry in Cycling 881
option for varying power-output levels is due to the large ranges of work rates performed by cyclists and triathletes during racing.28,29 In addition, using an incremental power-output test would be convenient to gather laboratory measures (ie, maximal oxygen uptake) along with bilateral lower-limb force/torque measures.
Conclusions
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Greater dominant-to-nondominant differences in peak torque were observed using instrumented pedals than with the instrumented crank system in power-output ranges of 100 to 250 W. Substantial differences in dominant-to-nondominant peak torques could only be assessed using instrumented pedals, with increased asymmetry observed at higher power outputs in favor of the dominant leg. Commercial instrumented crank systems are thus not recommended to assess crank-torque asymmetries during bilateral pedaling. Whenever possible, instrumented pedals should be used for torque- and lower-limb-asymmetry assessments. Acknowledgments The first author acknowledges Capes-Brazil for his PhD scholarship. The authors acknowledge AUT University for supporting this research. Thanks are given to the cyclists who participated in the study.
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11. Hull ML, Davis RR. Measurement of pedal loading in bicycling: I: instrumentation. J Biomech. 1981;14(12):843–856. PubMed doi:10.1016/0021-9290(81)90012-9 12. Candotti CT, Ribeiro J, Soares DP, de Oliveira AR, Loss JF, Guimarães ACS. Effective force and economy of triathletes and cyclists. Sports Biomech. 2007;6(1):31–43. PubMed doi:10.1080/14763140601058490 13. Neto CD, Schimidt G, Candotti CT, et al. Desenvolvimento de uma plataforma de força em pedal de ciclismo. Rev Bras Biomec. 2001;2(3):39–44. 14. Leirdal S, Ettema G. Pedaling technique and energy cost in cycling. Med Sci Sports Exerc. 2011;43(4):701–705. PubMed doi:10.1249/ MSS.0b013e3181f6b7ea 15. Marfell-Jones M, Olds T, Stewart A, Carter L. International Standards for Anthropometric Assessment. Potchefstroom, South Africa: ISAK; 2006. 16. Lucía A, Hoyos J, Pérez M, Santalla A, Chicharro JL. Inverse relationship between VO2max and economy/efficiency in world-class cyclists. Med Sci Sports Exerc. 2002;34(12):2079–2084. PubMed doi:10.1097/00005768-200212000-00032 17. Robinson RO, Herzog W, Nigg BM. Use of force platform variables to quantify the effects of chiropractic manipulation on gait symmetry. J Manipulative Physiol Ther. 1987;10(4):172–176. PubMed 18. Hopkins WG, Marshall SW, Batterham AM, Hanin J. Progressive statistics for studies in sports medicine and exercise science. Med Sci Sports Exerc. 2009;41(1):3–13. PubMed doi:10.1249/ MSS.0b013e31818cb278 19. Cohen J. Statistical Power Analysis for the Behavioral Sciences. Vol 1. 2nd ed. Hillsdale, NJ: Routledge Academic; 1988. 20. Gardner AS, Stephens S, Martin DT, Lawton E, Lee H, Jenkins D. Accuracy of SRM and Power Tap power monitoring systems for bicycling. Med Sci Sports Exerc. 2004;36(7):1252–1258. PubMed doi:10.1249/01.MSS.0000132380.21785.03 21. Edwards LM, Jobson SA, George SR, Day SH, Nevill AM. Wholebody efficiency is negatively correlated with minimum torque per duty cycle in trained cyclists. J Sports Sci. 2009;27(4):319–325. PubMed doi:10.1080/02640410802526916 22. Exell TA, Irwin G, Gittoes MJR, Kerwin DG. Implications of intralimb variability on asymmetry analyses. J Sports Sci. 2012;30(4):403– 409. PubMed doi:10.1080/02640414.2011.647047 23. Carpes FP, Diefenthaeler F, Bini RR, Stefanyshyn DJ, Faria IE, Mota CB. Influence of leg preference on bilateral muscle activation during cycling. J Sports Sci. 2011;29(2):151–159. PubMed doi:10.1080/02 640414.2010.526625 24. Carpes FP, Diefenthaeler F, Bini RR, Stefanyshyn D, Faria IE, Mota CB. Does leg preference affect muscle activation and efficiency? J Electromyogr Kinesiol. 2010;20(6):1230–1236. PubMed doi:10.1016/j.jelekin.2010.07.013 25. Edeline O, Polin D, Tourny-Chollet C, Weber J. Effect of workload on bilateral pedaling kinematics in non-trained cyclists. J Hum Mov Stud. 2004;46(6):493–517. 26. Kanchan T, Mohan Kumar TS, Pradeep Kumar G, Yoganarasimha K. Skeletal asymmetry. J Forensic Leg Med. 2008;15(3):177–179. PubMed doi:10.1016/j.jflm.2007.05.009 27. Ting LH, Raasch CC, Brown DA, Kautz SA, Zajac FE. Sensorimotor state of the contralateral leg affects ipsilateral muscle coordination of pedaling. J Neurophysiol. 1998;80(3):1341–1351. PubMed 28. Vogt S, Heinrich L, Schumacher YO, et al. Power output during stage racing in professional road cycling. Med Sci Sports Exerc. 2006;38(1):147–151. PubMed doi:10.1249/01. mss.0000183196.63081.6a 29. Abbiss CR, Quod MJ, Martin DT, et al. Dynamic pacing strategies during the cycle phase of an Ironman triathlon. Med Sci Sports Exerc. 2006;38(4):726–734. PubMed doi:10.1249/01. mss.0000210202.33070.55
www.IJSPP-Journal.com ORIGINAL INVESTIGATION
Assessment of Bilateral Asymmetry in Cycling Using a Commercial Instrumented Crank System and Instrumented Pedals Rodrigo R. Bini and Patria A. Hume The accuracy of commercial instrumented crank systems for symmetry assessment in cycling has not been fully explored. Therefore, the authors’ aims were to compare peak crank torque between a commercial instrumented crank system and instrumented pedals and to assess the effect of power output on bilateral asymmetries during cycling. Ten competitive cyclists performed an incremental cycling test to exhaustion. Forces and pedal angles were recorded using right and left instrumented pedals synchronized with crank-torque measurements using an instrumented crank system. Differences in right (dominant) and left (nondominant) peak torque and asymmetry index were assessed using effect sizes. In the 100- to 250-W power-output range, the instrumented pedal system recorded larger peak torque (dominant 55–122%, nondominant 23–99%) than the instrumented crank system. There was an increase in differences between dominant and nondominant crank torque as power output increased using the instrumented crank system (7% to 33%) and the instrumented pedals (9% to 66%). Lower-limb asymmetries in peak torque increased at higher power-output levels in favor of the dominant leg. Limitations in design of the instrumented crank system may preclude the use of this system to assess peak crank-torque symmetry. Keywords: bicycle, torque, dominance, pedal forces Bilateral cycling motion has usually been assessed assuming symmetry in force production and kinematics of lower limbs. However, differences in power output and mechanical work of the legs have ranged from 5% to 20% in uninjured cyclists and noncyclists.1 Conflicting results were reported comparing cyclists2–4 and noncyclists5 without clear relationships between pedaling cadence2 and power-output level5 in bilateral symmetry. Peak torque at the propulsive phase of crank revolution (ie, from 12 o’clock to 6 o’clock crank positions) has been reported as one of the most important predictors of performance during 40-km time trials,6 given that a large percentage of the force applied to the pedal in the sagittal plane can be translated into crank torque in this part of crank revolution.7 Therefore, cyclists should apply large crank torque on both cranks to enhance power output for a given pedaling cadence. Using peak torque as a measure of pedaling symmetry, authors have reported that differences between legs were significant at lower power-output levels (≤90% of maximal oxygen uptake) and decreased at higher power-output levels (ie, >91% of peak oxygen uptake) for 6 competitive cyclists.3,8 In contrast, another study did not show significant differences in mean torque computed during full crank revolution for 11 cyclists at different power-output levels (60–100% of maximal oxygen uptake).4 Therefore, it is unclear if crank-torque symmetry is related to power-output level. The potential reduction in asymmetries in torque at higher power-output levels may be due to an increased bilateral neural input by interhemispheric cortical communication to facilitate the excitability of both legs.1
Bini is with the Exercise Research Laboratory, Federal University of Rio Grande do Sul, Porto Alegre, Brazil. Hume is with Sport Performance Research Inst New Zealand, Auckland University of Technology, Auckland, New Zealand. Address author correspondence to Rodrigo Bini at bini. [email protected]. 876
Evaluation of bilateral asymmetry has gained popularity because some commercial devices provide right-to-left crank comparisons for torque and power output. One example is the SRM torque-analysis system, which enables the user to assess peak crank torque from dominant and nondominant lower limbs during cycling.9 The SRM power meter measures the deformation on the shafts of the crank set resulting from the torque applied on both cranks (ie, net crank torque). However, a recent study showed that the measures of peak torque from the SRM torqueanalysis system are only accurate for power output greater than 80% of maximal power output.10 Therefore, separate measures of dominant and nondominant crank torque are not accurate using this device, because the torques at dominant and nondominant cranks are computed as net torque (ie, torque from the contralateral leg diminishes torque from the ipsilateral leg). Using the commercial instrumented crank systems (ie, SRM torque-analysis system), it has been assumed that peak torque observed during the propulsive phase of crank revolution is exclusively affected by the ipsilateral leg,3 which may not be completely valid. Consequently, the accuracy of assessment of bilateral symmetry using instrumented crank systems may be compromised by the design of this device. Instrumented pedals have been able to provide crank-torque measurements independently for dominant and nondominant legs,11 which are expected to offer a more accurate measure of peak crank torque than instrumented crank systems. Therefore there is uncertainty on the accuracy of asymmetries in bilateral peak torque taken from instrumented crank systems (ie, SRM torque-analysis system). There is also a need to determine whether potential differences in bilateral peak-torque measurements taken from instrumented pedals and instrumented crank systems would be affected by changes in power-output level. Therefore, our aims were to compare peak crank torque between a commercial instrumented crank system and instrumented pedals and to assess the effect of power-output level in bilateral asymmetries during cycling. Our hypotheses were that the instrumented
Assessment of Bilateral Asymmetry in Cycling 877
crank system would underestimate bilateral asymmetries and that asymmetries would be diminished at higher power-output levels.
Methods Participants Ten cyclists (3 female and 7 male club riders) with competitive experience in cycling and/or triathlon were invited to participate in the study: age 30 ± 7 years, body mass 72.8 ± 13 kg, standing height 175 ± 12 cm, maximal oxygen uptake 55.6 ± 8.8 mL · kg–1 · min–1, peak power output 336 ± 77 W, and peak power per body mass 4.6 ± 6 W/kg). Lower-limb dominance assessed by the Waterloo Inventory indicated that all 10 cyclists were right-leg dominant. The ethics committee of AUT University approved the research protocol (AUTEC 10/56).
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Design A quantitative repeated-measures experimental design was used to collect data (cross-sectional).
Data Collection Force components (normal and anteroposterior) from a 2-dimensional pedal dynamometer custom developed for Look-type cleats12,13 were computed using the regression between 3 static load points (0, 5, and 10 kg) applied to the pedals and voltage output when R2 was greater than .99. Mechanical coupling between anteroposterior and normal loads was corrected using a gain matrix.14 Potentiometers attached to the pedal spindle (Vishay Spectrol model 357, Vishay Intertechnology, Malvern, PA, USA) were calibrated using a manual goniometer (Physio-Med Services, model 30 cm, Patterson Medical Ltd, Nottinghamshire, UK) set at 4 crank-to-pedal axle angles (0°, 90°, 180°, and 270°, taken twice for each crankto-pedal angle) to compute the relationship between voltage output and the measured angle. The calibration factors were defined when differences in voltage taken from both trials were lower or equal to 1% for each given crank-to-pedal angle. Body mass and height were measured according to International Society for the Advancement of Kinanthropometry protocols.15 Cyclists’ bicycle saddle height and horizontal position were measured to set up the stationary cycle ergometer (Velotron, Racermate Inc, Seattle, WA, USA). The cyclists performed an incremental cycling exercise on the cycle ergometer with 3 minutes of warmup at 100 W and pedaling cadence visually controlled at 90 ± 2 rpm. Power output was then increased to 150 W and continued increasing in a step profile of 25 W/min until cyclist exhaustion.16 A script was configured in the Velotron CS2008 software (Velotron, Racermate Inc, Seattle, WA, USA) for automatic control of the cycle-ergometer power output in a constant-workload mode. This configuration enabled a constant power output with cycleergometer resistance changing to balance for fluctuations in pedaling cadence. Gas exchanges were continuously sampled from a mixing chamber where samples were drawn into the oxygen and carbon dioxide analyzers for continuous measurement using a metabolic cart (TrueOne 2400, Parvo Medics, Salt Lake City, UT, USA). Analyzers for oxygen and carbon dioxide were calibrated according to manufacturer recommendations. Maximal power output and maximal oxygen uptake were defined as the highest power output measured during the test and as the highest oxygen uptake computed over a 15-second average of the data, respectively, for assessment of
cyclists’ performance and fitness level. All aforementioned procedures served as familiarization. After 2 to 7 days, cyclists returned to the laboratory at the approximate same time of day to perform the incremental test following the same procedures. Normal and anteroposterior forces were measured using a pair of strain-gauge instrumented pedals,12 with pedal-to-crank angle measured using angular potentiometers attached to the pedal spindle. Pedal-force signals passed through an amplifier (Signal conditioning unit, model RM-044, Applied Measurements, Mitcham, Australia) and, along with potentiometers, reed switch signals and SRM torque-analysis system signals (Schoberer Rad Meßtechnik, Jülich, Germany) were recorded using an analog-to-digital board (PCI-MIO-16XE-50, National Instruments, Austin, USA) at 600 Hz per channel using a custom-made script in Matlab (Mathworks Inc, Natick, MA, USA). Analog data were acquired between the 20th and the 40th seconds of each step of 50 W (ie, 100, 150, 200, 250 W, etc).
Data Analyses Pedal-to-crank angle measured by the potentiometers were converted into sine and cosine to compute tangential and radial forces on the cranks. A low-pass zero-lag Butterworth digital filter with cutoff frequency of 10 Hz was applied to the sine and cosine data from potentiometers to attenuate signal noise from the gap in potentiometer voltage readings.11 Crank torque was measured by the pedals using the tangential force on the cranks and crank length, as well as by the SRM torque-analysis system, as shown in Figure 1. A frequency-to-voltage conversion factor of 4 × 10–4 and frequency-to-torque factor gathered at the calibration trial were used to convert torque measurements from voltage to Nm. A reed switch attached to the bicycle frame detected the position of the crank in relation to the pedal revolution and enabled separate pedal forces and torque data for every crank revolution and for the propulsive (ie, from 12 o’clock to 6 o’clock crank positions) and recovery phases (ie, from 6 o’clock to 12 o’clock crank positions) for right and left cranks. Peak crank torques of right (dominant) and left (nondominant) cranks were determined when the crank was at the propulsive phase and at the recovery phase, respectively, using
Figure 1 — (A) Image of the instrumented pedal attached to the instrumented crank system. (B) Illustration of the locations of sensors for cranktorque measurement for the instrumented crank system and instrumented pedals. Vertical arrows indicate crank torque applied simultaneously by the ipsilateral and contralateral legs.
878 Bini and Hume
a clockwise motion of the crank as reference. Peak crank torque was averaged over 5 complete pedal revolutions for each crank on the instrumented pedals and the commercial instrumented crank system. Standard deviations for peak torque from dominant and nondominant limbs were computed from 5 crank cycles to report intralimb variability. Asymmetry index (AI%) was calculated as outlined by Robinson et al,17 using measures from dominant (D) and nondominant (ND) legs normalized by the average of dominant and nondominant measures:
AI% = {(D – ND)/[(D + ND/2]} × 100
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Statistical Analyses Errors of calibration of normal and anteroposterior components and potentiometers of the pedals were computed as average percentage differences in voltage due to calibration load (or angle for potentiometer) in relation to the output voltage. As an example, for the normal force of the right pedal, the difference in voltage from 0 kg to 5 kg was 0.1547 V, and the difference in voltage from 5 kg to 10 kg was 0.1544 V, resulting in 0.19% difference in voltage due to load application. Variation in pedaling cadence was computed by percentage differences across 5 crank revolutions. Peak torque for dominant and nondominant cranks and asymmetry index (mean and SD) were compared for instrumented pedals and the instrumented crank system. All variables were analyzed for the 100-W, 150-W, 200-W, 250-W, 300-W, and 350-W power outputs of the incremental test. Normality of distribution and sphericity were confirmed for all variables via the Shapiro-Wilk and Mauchly tests, respectively, after application of a logarithmic transformation using SPSS for Windows 16.0 (SPSS, New York, NY, USA). Mean percentage differences between dominant and nondominant peak torques and the asymmetry index from the instrumented crank system and the instrumented pedals were computed, and comparisons were conducted using Cohen effect sizes (ES). We used ESs opting for a threshold of large effects (ES = 1.0) for substantial
changes. This is a more conservative approach than previously described,18 but it would ensure a nonoverlapping in distribution of scores greater than 55%.19
Results Errors from calibration procedures were 0.19% and 0.68% for the normal force and 0.68% and 0.56% for anteroposterior force for the dominant and nondominant pedals, respectively. Error in pedal-to-crank angle of each potentiometer was 0.5%. Mean variation in pedaling cadence between cyclists was 1%, resulting in an estimated error from equipment of ~1.37% and ~1.74% for crank torque of the dominant and nondominant pedals, respectively. For the instrumented cranks, errors in instrumented crank measurements followed reports from previous studies.20 Greater peak torque was observed for dominant and nondominant pedals than for instrumented crank system, as shown in Figure 2 and Table 1. In general, large differences for dominant (31–48%) and nondominant (17–39%) peak crank torque between the commercial instrumented crank system and the instrumented pedals were observed at power-output ranges of 100 to 250 W. At higher power outputs (300 W and 350 W) there were moderate to trivial differences in peak crank torque comparing dominant with nondominant pedals. There was a trend for an increase in the difference between dominant and nondominant crank torques using the commercial instrumented crank system (7–33%) and the instrumented pedals (9–66%), but large differences were only found for the instrumented pedals at power outputs higher than 200 W. The instrumented pedals presented larger asymmetry indices than the commercial instrumented crank system at 250 W (see Table 1). Intralimb variability for peak crank torque from dominant and nondominant limbs (4–11%; see Table 2) was smaller than differences between dominant and nondominant peak torques for the instrumented crank system (5–33%) and for the instrumented pedals (9–66%).
Figure 2 — (A) dominant, (B) nondominant, and (C) dominant + nondominant crank torque measured by the pedals and by (D) the instrumented crank system (D). Data from 5 consecutive revolutions of the 10 cyclists at 200 W of workload and 90 rpm of pedaling cadence. Arrows indicate peak crank torque.
879
23 ± 8
31 ± 11
39 ± 14
53 ± 19
73 ± 25
100 W (n = 10)
150 W (n = 10)
200 W (n = 10)
250 W (n = 10)
300 W (n = 7)
350 W (n = 6)
D
75 ± 7
65 ± 8
56 ± 8
48 ± 7
41 ± 6
33 ± 5
CS vs D
3%; 0.1, T
19%; 0.9, M
31%; 1.6, L
36%; 2.0, L
43%; 2.4, L
48%; 3.0, L
CS
55 ± 18
45 ± 11
36 ± 9
28 ± 8
22 ± 7
18 ± 8
46 ± 14
46 ± 12
43 ± 12
39 ± 8
34 ± 10
30 ± 9
ND
21%; 0.6, M
1%; 0.1, T
17%; 0.7, M
27%; 1.3, L
35%; 1.4, L
39%; 1.4, L
CS vs ND
Nondominant Peak Torque (Nm)
33%; 0.8, M
17%; 0.5, M
8%; 0.3, S
10%; 0.3, S
5%; 0.1, T
7%; 0.2, T
CS
66%; 2.8, L
42%; 1.9, L
29%; 1.3, L
26%; 1.3, L
19%; 0.8, M
9%; 0.4, S
Pedals
Dominant vs Nondominant Peak Torque
Abbreviations: CS, crank system; D, dominant pedal; ND, nondominant pedal; T, trivial ES; S, small ES; M, moderate ES; L, large ES.
CS
17 ± 6
Power-output level
Dominant Peak Torque (Nm)
CS
27 ± 18
13 ± 20
5 ± 15
8 ± 17
4 ± 15
6 ± 17
51 ± 36
36 ± 33
28 ± 31
22 ± 30
20 ± 33
11 ± 28
Pedals
93%; 0.9, M
189%; 0.9, M
428%; 1.0, L
181%; 0.6, S
418%; 0.7, M
280%; 0.8, M
CS vs pedals
Asymmetry Index (%)
Table 1 Mean ± SD, Mean Percentage Differences, and Effect Sizes (ES) Comparing Both Systems (Commercial Instrumented Crank System and Instrumented Pedals) and Differences Between Dominant and Nondominant Cranks for Peak Torque and Asymmetry Index
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880 Bini and Hume
Table 2 Intralimb Variability in Crank Torque for the 6 Power Outputs From the Incremental Test From 10 Cyclists, Mean (SD) Peak torque, Nm Power output
Dominant
Nondominant
100 W (n = 10) 150 W (n = 10) 200 W (n = 10) 250 W (n = 10) 300 W (n = 7) 350 W (n = 6)
1.4 (4%) 2.2 (5%) 2.7 (6%) 3.4 (6%) 5.1 (8%) 8.1 (11%)
2.2 (7%) 1.8 (5%) 2.2 (6%) 3.2 (8%) 3.2 (7%) 3.7 (8%)
Note: SD for peak torque from 5 crank cycles for dominant and nondominant limbs.
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Discussion Our study compared dominant with nondominant peak crank torque measured by a commercial instrumented crank system (ie, SRM torque-analysis system) and instrumented pedals during bilateral cycling at varying power-output levels. The reason for these comparisons was based on the increasing use of the instrumented crank system for the assessment of asymmetries in cycling3,8 and potentially because the commercial instrumented crank system may underestimate peak crank torque and asymmetry.10 Lower peak torque was observed in this study for the commercial instrumented crank system compared with the instrumented pedals for the same power-output level. Greater asymmetries were observed at higher power-output levels, but large differences between dominant and nondominant crank torque were only observed using the instrumented pedals. The asymmetry index was greater than 20% (usually reported for uninjured cyclists1) for the commercial instrumented crank system only at 350 W and for the instrumented pedals at power outputs greater than 150 W. The primary reason for these differences between systems is related to the electronic characteristics of each system. The commercial instrumented crank system, as outlined in Figure 1, is designed to measure the deformation on the shafts of the crank set due to the torque applied on both cranks. Using the example highlighted in Figure 1, if a cyclist applied 20 Nm of torque (clockwise) with the ipsilateral leg and 5 Nm of torque (anticlockwise) with the contralateral leg, the commercial instrumented crank system would record 15 Nm of torque. Therefore, the torque at dominant and nondominant cranks is computed as a net torque (ie, torque from the contralateral leg diminishes torque from the ipsilateral leg). Instrumented pedals measure the force on the pedal surface (eg, normal and anteroposterior components) and compute crank torque independently using pedal-to-crank angles.11 This is a more accurate approach because forces from each leg are measured separately, reducing contralateral-to-ipsilateral effects on crank-torque measures. Therefore, care should be taken if instrumented cranks are used for pedaling-technique assessment due to possible interference in the data of each leg caused by the opposite leg, as previously conducted.21 Intralimb variability has been recently suggested to play a role in bilateral asymmetries in running.22 However, intralimb variability in peak torque (4–11%) was smaller in our study than differences between dominant and nondominant peak torque (5–66%). Additional concern has been raised due to the use of the asymmetry index to identify bilateral differences in kinetic variables during dynamic tasks.22 Large between-subjects variability in asymmetry indices (for the instrumented crank system and the instrumented
pedals) resulted in less pronounced ESs comparing instrumented cranks with pedals than using raw crank-torque data. Reduced ESs for comparison between instrumented cranks and pedal-asymmetry indices reinforce the limitation of asymmetry indices to detect bilateral differences in kinetic variables during cycling. Higher power outputs resulted in greater bilateral differences assessed by instrumented pedals that are contrary to previous findings.3,4,8 Cyclists were observed to reduce differences in peak torque at higher power outputs3,8; however, peak torque was measured using an instrumented crank system in those studies. Another study did not observe effects from power-output level in full-crankrevolution average torque symmetry in cyclists using instrumented pedals.4 Reductions in asymmetries in crank torque at higher power outputs have been hypothesized due to a potential increased bilateral neural input by interhemispheric cortical communication to facilitate the excitability of both legs.1 However, studies assessing muscle activation in cyclists during bilateral cycling exercise at increasing power outputs did not report differences in lower-limb muscle activation comparing both legs.23 Muscle activation during single-leg cycling did not differ when cyclists’ dominant and nondominant legs were compared,24 which suggests that lower-limb neural drive may not differ between legs. Substantial differences in cycling efficiency have not been observed in cyclists during single-leg cycling, suggesting that contributions to efficiency from independent legs are similar.24 Potential differences in joint motion25 from bilateral asymmetries in bone dimensions could be a reason for observed asymmetries in pedaling kinetics. It may be the case that asymmetries are related to changes in bone dimensions due to asymmetrical load applied to the skeleton during bone growth.26 Further research should shed light on why some cyclists present larger crank-torque asymmetries than others and how this would affect their performance and injury risk. Lower-limb dominance may have played a role in increasing asymmetries at higher power outputs. Evidence suggests that the kicking dominant leg contributed significantly more to average crank power than the nondominant leg,2 which is in line with our results. It has been previously shown that pedaling asymmetries are highly variable across different days of assessment and that asymmetries depend on exercise condition.2,5 Further research should assess bilateral muscle activation and joint kinetics in cyclists with similar levels of crank-torque asymmetry to compute individual joint contributions to crank torque from right and left legs. Some limitations may have affected the results of our study. Trials using single-leg cycling could have been conducted to isolate the influence of the contralateral leg in torque readings from the instrumented crank system. During single-leg cycling, torque measured by the instrumented crank system should not be affected by the contralateral leg; therefore, crank-torque measures would be isolated to the ipsilateral leg.27 Although this approach is not ecologically optimal, it would shed light on the effects of contralateral resistive torque on the data from the instrumented crank system and would provide a practical use of the instrumented crank system for measurement of bilateral asymmetries. Combined fatigue and power-output changes during the incremental test may have affected measures of peak torque and bilateral asymmetries. Randomizing power outputs would isolate for fatigue and work-rate effects in crank-torque asymmetries.
Practical Applications For coaches and cyclists, assessing bilateral asymmetries should preferably be conducted using instrumented pedals rather than instrumented crank systems at varying power-output levels. The
Assessment of Bilateral Asymmetry in Cycling 881
option for varying power-output levels is due to the large ranges of work rates performed by cyclists and triathletes during racing.28,29 In addition, using an incremental power-output test would be convenient to gather laboratory measures (ie, maximal oxygen uptake) along with bilateral lower-limb force/torque measures.
Conclusions
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Greater dominant-to-nondominant differences in peak torque were observed using instrumented pedals than with the instrumented crank system in power-output ranges of 100 to 250 W. Substantial differences in dominant-to-nondominant peak torques could only be assessed using instrumented pedals, with increased asymmetry observed at higher power outputs in favor of the dominant leg. Commercial instrumented crank systems are thus not recommended to assess crank-torque asymmetries during bilateral pedaling. Whenever possible, instrumented pedals should be used for torque- and lower-limb-asymmetry assessments. Acknowledgments The first author acknowledges Capes-Brazil for his PhD scholarship. The authors acknowledge AUT University for supporting this research. Thanks are given to the cyclists who participated in the study.
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